This invention relates to the technical field of imaging with light-sensitive thermally developable recording materials, particularly to thermal developing apparatus that form color on exposed light-sensitive thermally developable recording materials by thermal development.
Images obtained by medical diagnoses such as those using ultrasonic imaging instruments, computerized tomography (CT), nuclear magnetic resonance (NMR) and X-ray imaging techniques are conventionally recorded on silver salt photographic materials, developed and processed into hard copies. Silver salt photographic materials have the advantage of yielding high-quality images but on the other hand they require wet processing involving color development, bleaching and rinsing and hence suffer from the problem of taking much time and labor in overall development processing. In addition, the maintenance of the developing machine (processor) is a time-consuming job. Under the circumstances, it is desired to produce hard copies by an image forming method that does not require any wet development processing.
Thermal image recording is a known technique capable of forming images without wet processes. As is well known, thermal image recording uses a thermal head having a glaze comprising heating elements arranged in one direction (main scanning direction) and with the glaze pressed against a thermal recording material, the two are moved in relative directions, one being the main scanning direction and the other being a direction normal to it, and image is recorded on the thermal recording material by generating heat from the specific heating elements as determined by the image to be recorded.
The quality of the image obtained by thermal recording has recently seen marked improvements and an extension of its use from the conventional ultrasonic imaging to other applications such as CT, NMR and X-ray imaging techniques that require big and high-quality images is being reviewed. However, in such aspects as recording density and tone reproduction, the image formed by exposing silver salt photographic materials to optical beams or the like is still advantageous.
This problem has been solved by an attractive method of image recording with light-sensitive thermally developable recording materials which form color by exposure and heating or which have color formed in the non-exposed area by heating with care being taken to avoid color formation by exposure. Such light-sensitive thermally developable recording materials are imagewise exposed to optical beams or the like to form a latent image and subsequently heated to form color in either the exposed area or the non-exposed area, thereby producing a hard copy with a recorded image. According to image recording with such light-sensitive thermally developable recording materials, high-definition and high-quality images can be formed by optical beam scanned exposure or the like but without performing any steps in wet development processing.
A thermal developing apparatus is necessary to perform thermal development in the process of image formation with the above-described light-sensitive thermally developable recording material (which is hereinafter referred to simply as the "recording material") and a known type of such apparatus comprises a heating drum having a built-in heater such as a halogen lamp and an endless belt wrapped around about two-thirds of the circumference of the heating drum. The recording material having a latent image formed by exposure is inserted between the heating drum and the endless belt and with the heating drum rotating at a specified speed, the recording material is pinched between the heating drum and the endless belt and transported as it is kept in close contact with the heating drum so that thermal development is achieved by heating at a specified temperature for a specified time.
The more closely the recording material is kept in contact with the heating drum, the higher the heat efficiency with which it is developed thermally. The color density of the recording material depends on the quantity of heat imparted from thermal development, so in order to form a high-quality image, a specified quantity of heat must be applied uniformly to the entire surface of the recording material. To this end, the surface of the heating drum is made flat enough by using metals or coating them with resins such as Teflon.
However, if dust or other foreign matter enters between the recording material and the heating drum during thermal development, the recording material "floats" microscopically and the efficiency of heat transfer in the affected area will decrease. As the result, the quantity of heat being imparted to the recording material by thermal development varies from place to place and uneven densities occur due to unevenness in thermal development. Similarly, if a flaw occurs on the surface of the heating drum, not only the damaged area of the recording material but also its nearby areas are deteriorated in heating efficiency and again the quantity of heat being imparted to the recording material by thermal development varies from place to place and uneven densities occur due to unevenness in thermal development.
The light-sensitive thermally developable recording material comprises a variety of layers including a support (base material) such as a polyethylene terephthalate (PET) film, an image forming layer having a color forming agent and a sensitizing agent dispersed and/or dissolved in a binder, a protective layer to protect the image forming layer, a back layer formed on the other side of the support opposite to the image forming layer.
In some types of the recording material, any components contained in the respective layers evaporate and/or decompose during thermal development to produce gas. The gas may often adversely affect the color formation on the recording material (image forming layer). Especially, the color density often varies abnormally due to the gas confined between the recording material and the heating drum during thermal development.